The reference assay in diagnostics is the so-called “ELISA” assay. With this assay it is possible to detect or determine an analyte in a biological fluid. In general, the wells of a microtitre plate are coated (by essentially hydrophobic interaction) with a capturing element (for example an antibody) capable of binding specifically to an analyte (in particular an antigen) that is being sought. The test solution is then deposited in the wells of the microplate, and if the molecule sought is present, it binds specifically to the capturing element.
A tracer element, also capable of binding to the molecule being sought, is added to the wells. This tracer element can be for example an enzyme that catalyses the formation of a coloured product, so that this tracer element can be quantified by colorimetry.
The ELISA assay is well suited to automation. However, the surfaces conventionally used for this assay (of polystyrene) cannot easily immobilize hydrophilic molecules, such as negatively charged polysaccharides, or those of low molecular weight. It has also been proposed for example to conjugate a polysaccharide with a polylysine so as to be able to fix it to a polystyrene surface, utilizing the adsorption capabilities of polylysine (Leinonen & Frasch, Infect Immun., 38(3): 1203-7, 1982).
The ELISA assay also has limitations connected with the surface charge, because the interactions are generally more specific with negatively-charged surfaces (Graves, J. Immunol. Methods, 111(2): 157-66, 1988), whereas the surfaces of plastics are hydrophobic.
Moreover, the standard ELISA assay is limited to mono-parametric analyses, which means that a single piece of information is available per test and per sample. When several analyses are required on the same sample, it is necessary to carry out several assays of the ELISA type, preferably in parallel, either conventionally, or miniaturized, in a device known as a “biochip”. Very few biochips are currently marketed, and the devices currently on offer mainly use glass microscope slides as substrates. Such a substrate is poorly suited to mass use.
On the other hand, plastic substrates are poorly suited to fabrication of biochips. Unsatisfactory signal/noise ratios are generally obtained in direct adsorption on the substrate, owing to the considerable nonspecific adsorption of the analytes. Hydrophobic plastic surfaces also have the drawback that they denature proteins, and therefore cannot be used without modification.
However, the possibilities of functionalization of plastics by chemical modification of the polymers in question are very limited, as few chemical groups can be introduced. Moreover, a minor change in the composition of the polymers, or even variations in composition from one batch to the next, would lead to differences in properties of the functionalized substrates.
Moreover, important constraints are encountered during printing of biochips, as it is difficult to deposit nanodrops reproducibly (so as to obtain deposits that are well defined and regular) on such substrates. The most practical method for depositing drops in this context, contact printing with needles, is a technical challenge, so that the contactless deposition technique of the piezoelectric type is generally used. However, in view of its slowness, the latter is not very suitable for industrial production.
Some examples of functionalization of 96-well plastic plates have been proposed. Thus, the plasma technique was suggested in Marson, Robinson et al., Glycobiology 19(2): 1537-46 (2009), for preparing sugar-based chips. This technique poses the difficulty of preserving the functionality of the sugar after immobilization. Another example is based on the use of streptavidin for coating plastic surfaces (Gehring, Albin et al., Anal. Bioanal. Chem., 5 Apr. 2008).
Document EP 1 953 555 describes a device comprising a plastic substrate covered with a metallic film on which a physiologically active substance and a compound for creating hydrogen bonds are immobilized.
Document EP 2 030 677 describes a biosensor comprising a substrate covered with a metallic film, by means of which an anionic polymer is fixed.
These documents do not disclose a direct interaction between the polymer and a plastic substrate.
There is therefore still a need to supply devices for detecting analytes that are easy to manufacture, robust, easy to use, and/or that make it possible to detect a large number of analytes in parallel.